Symplectic Lefschetz Fibrations and the Geography Of

Total Page:16

File Type:pdf, Size:1020Kb

Symplectic Lefschetz Fibrations and the Geography Of Symplectic Lefschetz fibrations and the geography of symplectic 4-manifolds Matt Harvey January 17, 2003 This paper is a survey of results which have brought techniques from the theory of complex surfaces to bear on symplectic 4-manifolds. Lefschetz fibrations are defined and some basic examples from complex surfaces discussed. Two results on the relationship between admitting a symplectic structure and admitting a Lefschetz fibration are explained. We also review the question of geography: 2 which invariants (c1, c2) occur for symplectic 4-manifolds. Constructions of simply connected symplectic manifolds realizing certain of these pairs are given. 1 Introduction It is possible to answer questions about a broad class of 4-manifolds by using the fact that they admit a special kind of structure called a Lefschetz fibration: a map from the manifold to a complex curve whose fibers are Riemann surfaces, some of them singular. The interesting data in such a fibration is the number of singular fibers and the types of these singular fibers. The types can be specified by monodromies (elements of the mapping class group of a regular fiber) associated to loops around the critical values of the fibration map in the base curve. Lefschetz fibrations should, at least in theory, frequently be applicable to ques- tions about symplectic 4-manifolds, in light of a theorem of Gompf that a Lef- schetz fibration on a 4-manifold X whose fibers are nonzero in homology gives a symplectic structure on X and a theorem of Donaldson that, after blowing up a finite number of points, every symplectic 4-manifold admits a Lefschetz fibration. After giving some examples from the category of complex surfaces and dis- cussing the above results on Lefschetz fibrations, we state and sketch a proof of Gompf’s theorem that fiber sums can be carried out along codimension 2 sym- 1 plectic submanifolds so that the resulting manifold will still carry a symplectic structure. Next is a brief introduction to the geography problem. The interesting part of this problem in the symplectic category is the question of existence of symplec- tic manifolds which are not homeomorphic to any complex surface. We will see that such manifolds do exist, and in fact all possible invariants of spin symplec- tic manifolds lying below the so-called Noether line are easy to realize using Gompf’s symplectic fiber sum. There are a few conventions used throughout the paper. The word curve always means complex curve (dimR = 2), but surface means closed 2-manifold. A complex surface will always be explicitly referred to as such. Coefficients for homology and cohomology are taken in Z when not otherwise specified. The Euler class e(ξ) of a principal U(1) = SO(2) bundle ξ is the same as the first Chern class. We may evaluate e(ξ)[S2] for such a bundle over the 2-sphere and speak of the Euler number. This can be viewed in terms of building total space of the associated disc bundle to the given principal bundle from two copies of D2 × D2. In particular, in the first factor of this product, the construction is that of S2 as the union of the “northern” and “southern” hemispheres along the “equator.” The fibers in the second factor are identified by assigning an element of SO(2) to each point of the equator. By a diffeomorphism, we can choose the element of SO(2) assigned to some fixed base point on the equator to be the identity. The identifications over the equator then give a map S1 → SO(2). The isomorphism type of the principal bundle depends only on the homotopy class ∼ of this map, which is an element of π1(SO(2)) = Z called the Euler number of the bundle. PD(·) is used in various contexts to denote the Poincar´edulaity ∼ n−k isomorphism Hk(X) = H (X) for compact manifolds. We sometimes abuse notation and identify the latter group with the deRham cohomology. 2 Building blocks in the complex category The following definition will be needed later, but it also provides one of the most basic examples of a 4-manifold admitting a Lefschetz fibration. (This particular one has no critical points and no singular fibers.) Definition 2.1 A ruled surface is a compact complex surface S with a map 1 π : S → C to a complex curve C such that every fiber is a complex line CP . 2 2.1 Lefschetz fibrations The structure of Lefschetz fibrations comes from a decomposition of complex surfaces called a Lefschetz pencil. n If S ⊂ CP is a complex surface, then a generic codimension 2 linear subspace n A ⊂ CP meets S in some finite set of points B. We may then consider two n codimension 1 linear subspaces of CP which both contain the subspace A but otherwise are generic. We can specify these subspaces as the zero sets of two linear homogeneous polynomials V (p0) and V (p1). 1 Now for fixed t = [t0 : t1] ∈ CP , consider the variety Lt = V (t0p0 + t1p1). This n will be a codimension 1 linear subspace of CP containing A. By a dimension count, we see that Lt intersects our original surface S in a complex curve, 1 which may be singular. Choosing s, t ∈ CP , note that Ls ∩ Lt = A so that (Ls ∩ S) ∩ (Lt ∩ S) = B. That is, all of the curves obtained above intersect exactly in the finite set of points B. After blowing up each point of B, we obtain 2 1 1 a well-defined map X#|B|CP → CP by sending each point to that t ∈ CP specifying the curve Lt ∩ S on which it lies. Definition 2.2 A Lefschetz fibration on a 4-manifold X is a map π : X → Σ where Σ is a closed 2-manifold having the following properties: 1. The critical points of π are isolated. 2. If P ∈ X is a critical point of π then there are local coordinates (z1, z2) on X and z on Σ with P = (0, 0) and such that in these coordinates π is 2 2 given by the complex map z = π(z1, z2) = z1 + z2 . In terms of this descrtiption of the critical points, a Lefschetz fibration can be thought of as a kind of complex analogue of a Morse function. Note that if the 4-manifold X is closed (∂X = ∅ and X is compact) then the generic fiber F of π is a closed 2-manifold, so a Riemann surface of some genus. Part of the utility of Lefschetz fibrations is that this simple property means that X can be reconstructued once we know how the fibers are identified if one travels around some loop in Σ \ C where C ⊂ Σ is the set of critical values of π. That is, Lefschetz fibrations admit a combinatorial description in terms of their monodromy representation ρ : π1(Σ \ C) → π0(Diff(F )) from the fundamental group of Σ \ C into the group of homotopy classes of self-diffeomorphisms of F . The latter group is often called the mapping class group. 3 2.2 Examples 2 2 A very simple example of a Lefschetz fibration is a map from CP #CP → 1 2 CP . One begins with a point P ∈ CP and two generic lines containing this point, which we can take to be the vanishing sets V (p0) and V (p1) of two 2 independent degree 1 polynomials. For any point Q ∈ CP \ P there is a unique line V (pQ) containing both P and Q given as the vanishing set of another degree 1 polynomial. Some easy linear algebra shows that one can write pQ = t0p0+t1p1 1 for some [t0 : t1] ∈ CP . The map Q 7→ [t0 : t1] gives the desired fibration on 2 CP \ P . Since we can view blowing up the point P as replacing the point P by the set of all lines passing through P , we obtain a well-defined fibration 2 2 1 2 2 CP #CP → CP . Note that this shows CP #CP is a ruled surface since the 1 fiber over a point [t0 : t1] is V (t0p0 + t1p1), a CP . 2 One can play a similar game starting with points P1,...,P9 ∈ CP given as the intersection V (p0)∩V (p1), where p0 and p1 are two generic homogeneous cubics in three variables. The nine intersection points are in general position. We will be interested in vanishing sets of polynomials of the form 3 3 3 2 2 a1X + a2Y + a3Z + a4X Y + a5X Z+ 2 2 2 2 a6Y Z + a7Z X + a8Z Y + a9Y X + a10XY Z. 2 Picking a point Q ∈ CP \{P1,...,P9} and asking that its coordinates [X : Y : Z] satisfy the above equation gives one linear relation on the ai. In the same way P1,...,P9 give 9 more. Generically, we obtain in this way a homogeneous cubic which (by more linear algebra) can be written as pQ = t0p0 + t1p1. Note 0 that all points Q ∈ V (pQ) give the same [t0 : t1]. Again, by viewing the blow-up of each of the Pi as replacing that point by the set of all points passing through 2 2 1 it, we obtain a map CP #9CP → CP whose generic fibers are elliptic curves 2 in CP , which are topologically the two-dimensional torus T 2. A holomorphic map π : S → C from a complex surface to a complex curve whose generic fibers are nonsingular elliptic curves is called an elliptic fibration. The above is an example known as E(1). There is also the notion of C∞ elliptic fibration which is roughly a map from a 4-manifold to a 2-manifold which is locally diffeomorphic to a holomorphic elliptic fibration by diffeomorphisms of open subsets of S and C commuting with the given maps S → C.
Recommended publications
  • Lefschetz Fibrations and Symplectic Structures 17
    Lefschetz fibrations and symplectic structures Jan-Willem Tel Universiteit Utrecht A thesis submitted for the degree of Master of Science February 2015 Supervisor: Second examiner: Dr. Gil Cavalcanti Prof. Dr. Marius Crainic Lefschetz fibrations and symplectic structures Jan-Willem Tel Abstract In this thesis, we study a relation between symplectic structures and Lefschetz fibrations to shed some light on 4-manifold theory. We introduce symplectic manifolds and state some results about them. We then introduce Lefschetz fibrations, which are a generalization of fiber bundles, and discuss them briefly to obtain an intuitive understanding. The main theorem of this thesis is a result obtained by Gompf [4]. It provides a way to construct a symplectic structure on a general Lefschetz fibration with homologeously nonzero fiber. We also discuss a generalization of this, achieved by Gompf [6], that generalizes this result to arbitrary even dimensions. Contents Chapter 1. Introduction 1 Chapter 2. Symplectic manifolds 2 1. Definition and basic properties 2 2. Compatibility with complex structures 4 3. Blow-ups 6 4. E(1) 7 Chapter 3. The symplectic normal sum 8 1. Construction and proof 8 2. Application: examples of symplectic manifolds 10 Chapter 4. Lefschetz fibrations 12 1. A generalization of fiber bundles 12 2. Basic properties and topology of Lefschetz fibrations 13 3. The exact sequence of a Lefschetz fibration 14 Chapter 5. The main theorem in 4 dimensions 15 1. Main theorem: Lefschetz fibrations and symplectic structures 15 2. Lefschetz pencils 19 Chapter 6. Generalization 20 1. Hyperpencils 20 2. The main theorem in 2n dimensions 21 Part (1) of the proof 21 Part (2) of the proof 25 Bibliography 29 iii CHAPTER 1 Introduction Symplectic geometry has been an important subject of research for the past decades.
    [Show full text]
  • Minimality of Symplectic Fiber Sums Along Spheres 3
    MINIMALITY OF SYMPLECTIC FIBER SUMS ALONG SPHERES JOSEF G. DORFMEISTER Abstract. In this note we complete the discussion begun in [24] con- cerning the minimality of symplectic fiber sums. We find that for fiber sums along spheres the minimality of the sum is determined by the cases 2 V V C discussed in [27] and one additional case: If X# Y = Z# CP 2 P with 2 VCP 2 an embedded +4-sphere in class [VCP 2 ] = 2[H] ∈ H2(CP , Z) and Z has at least 2 disjoint exceptional spheres Ei each meeting the sub- manifold VZ ⊂ Z positively and transversely in a single point with [Ei] · [VX ] = 1, then the fiber sum is not minimal. Contents 1. Introduction 1 2. Preliminaries 4 2.1. Symplectic fiber sum 4 2.2. Notation 4 2.3. Splitting of classes under fiber sums 5 2.4. Minimality of 4-Manifolds 5 2.5. Gromov-Witten Invariants and Fiber Sums 6 3. Symplectic Sums along Spheres 11 4. Minimality of Sums along Spheres 15 4.1. Minimality of Sums with S2-bundles 15 4.2. Vanishing of Relative Invariants in CP 2 15 4.3. Symplectic Sums for Embedded Curves 16 4.4. Minimality of Sums with CP 2 17 arXiv:1005.0981v1 [math.SG] 6 May 2010 4.5. Rational Blow-down 20 References 21 1. Introduction The symplectic fiber sum has been used to great effect since its discovery to construct new symplectic manifolds with certain properties. In four di- mensions this has focused on manifolds with symplectic Kodaira dimension 1 or 2.
    [Show full text]
  • Arxiv:Math/0504345V3 [Math.GT] 7 Jun 2005 H Eodato Rtflyakolde Upr Rmthe from Support Acknowledges Gratefully Author Second the [6]
    ON SYMPLECTIC 4-MANIFOLDS WITH PRESCRIBED FUNDAMENTAL GROUP SCOTT BALDRIDGE AND PAUL KIRK Abstract. In this article we study the problem of minimizing aχ + bσ on the class of all symplectic 4–manifolds with prescribed fundamental group G (χ is the Euler characteristic, σ is the signature, and a,b ∈ R), focusing on the important cases χ, χ + σ and 2χ + 3σ. In certain situations we can derive lower bounds for these functions and describe symplectic 4-manifolds which are minimizers. We derive an upper bound for the minimum of χ and χ + σ in terms of the presentation of G. 1. Introduction Pick a finitely presented group G and let M(G) denote the class of closed symplectic 4-manifolds M which have π1(M) isomorphic to G. The existence of a symplectic M with given fundamental group G was demonstrated by Gompf [6]. In this article we study the problem of finding minimizers in M(G) where minimizing is taken with regard to the Euler characteristic χ, following the approach introduced by Hausmann and Weinberger in [8] for smooth 4- manifolds. There are two aspects to this problem. Finding lower bounds to χ(M) for M ∈ M(G) addresses the question “How large must a symplectic manifold with fundamental group G be?” The other aspect of the problem is finding efficient and explicit constructions of symplectic manifolds with a given fundamental group. Our main general result concerning upper bounds is Theorem 6, which states: arXiv:math/0504345v3 [math.GT] 7 Jun 2005 Theorem 6. Let G have a presentation with g generators x1, · · · ,xg and r relations w1, · · · , wr.
    [Show full text]
  • (2015) Pp. 87-115: Geography of Symplectic 4- and 6-Manifolds
    http://topology.auburn.edu/tp/ Volume 46, 2015 Pages 87{115 http://topology.nipissingu.ca/tp/ Geography of Symplectic 4- and 6-Manifolds by Rafael Torres and Jonathan Yazinski Electronically published on May 5, 2014 Topology Proceedings Web: http://topology.auburn.edu/tp/ Mail: Topology Proceedings Department of Mathematics & Statistics Auburn University, Alabama 36849, USA E-mail: [email protected] ISSN: 0146-4124 COPYRIGHT ⃝c by Topology Proceedings. All rights reserved. http://topology.auburn.edu/tp/ http://topology.nipissingu.ca/tp/ Volume 46 (2015) Pages 87-115 E-Published on May 5, 2014 GEOGRAPHY OF SYMPLECTIC 4- AND 6-MANIFOLDS RAFAEL TORRES AND JONATHAN YAZINSKI Abstract. The geography of minimal symplectic 4-manifolds with arbitrary fundamental group and symplectic 6-manifolds with abelian fundamental group of small rank, and with arbitrary fundamental group are addressed. 1. Introduction The geography problem for simply connected irreducible symplectic 4-manifolds with negative signature has been settled in its entirety [20], [16], [36], [37], [1], [15], [8], [9], [3], [4]. Much less is known about the non simply connected realm. It has been observed that the new construction techniques [20], [32], [5] prove to be rather effective when one wishes to study the symplectic geography problem for a variety of choices of fundamental groups [20], [9], [3], [4], [46], [45]. Concerning symplectic 4- manifolds with prescribed fundamental group, we obtained the following theorem. Theorem 1.1. Let G be a group with a presentation consisting of g gen- erators and r relations. Let e and σ be integers that satisfy 2e + 3σ ≥ 0; e + σ ≡ 0 mod 4, and e + σ ≥ 8, and assume σ ≤ −1.
    [Show full text]
  • Symplectic Geometry
    CHAPTER 3 Symplectic Geometry Ana Cannas da Silva 1 Departamento de Matem(ttica, Instituto Superior T~cnico, 1049-001 Lisboa, Portugal E-mail: acannas @math.ist.utl.pt; acannas @math.princeton.edu Contents Introduction ..................................................... 81 1. Symplectic manifolds .............................................. 82 1.1. Symplectic linear algebra ......................................... 82 1.2. Symplectic forms ............................................. 84 1.3. Cotangent bundles ............................................. 85 1.4. Moser's trick ................................................ 88 1.5. Darboux and Moser theorems ...................................... 90 1.6. Symplectic submanifolds ......................................... 91 2. Lagrangian submanifolds ............................................ 93 2.1. First Lagrangian submanifolds ...................................... 93 2.2. Lagrangian neighborhood theorem .................................... 95 2.3. Weinstein tubular neighborhood theorem ................................ 96 2.4. Application to symplectomorphisms ................................... 98 2.5. Generating functions ........................................... 99 2.6. Fixed points ................................................ 102 2.7. Lagrangians and special Lagrangians in C n ............................... 104 3. Complex structures ............................................... 108 3.1. Compatible linear structures ....................................... 108 3.2.
    [Show full text]
  • STABILITY and EXISTENCE of SURFACES in SYMPLECTIC 4-MANIFOLDS with B+ = 1
    STABILITY AND EXISTENCE OF SURFACES IN SYMPLECTIC 4-MANIFOLDS WITH b+ = 1 JOSEF G. DORFMEISTER, TIAN-JUN LI, AND WEIWEI WU Abstract. We establish various stability results for symplectic sur- faces in symplectic 4¡manifolds with b+ = 1. These results are then applied to prove the existence of representatives of Lagrangian ADE- con¯gurations as well as to classify negative symplectic spheres in sym- plectic 4¡manifolds with · = ¡1. This involves the explicit construc- tion of spheres in rational manifolds via a new construction technique called the tilted transport. MSC classes: 53D05, 53D12, 57R17, 57R95 Keywords: smooth spheres, symplectic spheres, Lagrangian submanifolds Contents 1. Introduction 2 2. A Technical Existence Result 7 2.1. Generic Almost Complex Structures 8 2.2. Existence of Symplectic Submanifold 10 2.3. The Relative Symplectic Cone 14 3. Stability of symplectic curve con¯gurations 16 3.1. Smoothly isotopic surfaces under deformation{Theorem 1.3 16 3.2. Existence of Di®eomorphic surfaces 18 4. Lagrangian ADE-con¯gurations 20 4.1. Conifold transitions and stability of Lagrangian con¯gurations 20 4.2. Existence 23 5. Spheres in Rational Manifolds 25 5.1. Homology classes of smooth ¡4 spheres 25 5.2. Tilted transport: constructing symplectic (-4)-spheres 33 5.3. Spheres with self-intersection ¡1,¡2 and ¡3 37 5.4. Discussions 39 6. Spheres in irrational ruled manifolds 41 6.1. Smooth spheres 41 6.2. Symplectic spheres 44 JD was partially supported by the Simons Foundation #246043. TJL was partially supported by was partially supported by NSF Focused Research Grants DMS-0244663 and NSF grant DMS-1207037.
    [Show full text]
  • MONODROMY INVARIANTS in SYMPLECTIC TOPOLOGY This Text
    MONODROMY INVARIANTS IN SYMPLECTIC TOPOLOGY DENIS AUROUX This text is a set of lecture notes for a series of four talks given at I.P.A.M., Los Angeles, on March 18-20, 2003. The ¯rst lecture provides a quick overview of sym- plectic topology and its main tools: symplectic manifolds, almost-complex structures, pseudo-holomorphic curves, Gromov-Witten invariants and Floer homology. The second and third lectures focus on symplectic Lefschetz pencils: existence (follow- ing Donaldson), monodromy, and applications to symplectic topology, in particular the connection to Gromov-Witten invariants of symplectic 4-manifolds (following Smith) and to Fukaya categories (following Seidel). In the last lecture, we o®er an alternative description of symplectic 4-manifolds by viewing them as branched cov- ers of the complex projective plane; the corresponding monodromy invariants and their potential applications are discussed. 1. Introduction to symplectic topology In this lecture, we recall basic notions about symplectic manifolds, and briefly review some of the main tools used to study them. Most of the topics discussed here can be found in greater detail in standard graduate books such as [McS]. 1.1. Symplectic manifolds. De¯nition 1.1. A symplectic structure on a smooth manifold M is a closed non- degenerate 2-form !, i.e. an element ! ­2(M) such that d! = 0 and v T M 0 , ¶ ! = 0. 2 8 2 ¡ f g v 6 For example, R2n carries a standard symplectic structure, given by the 2-form !0 = dxi dyi. Similarly, every orientable surface is symplectic, taking for ! any non-vanishing^ volume form.
    [Show full text]
  • Tropical Enumeration of Curves in Blowups of the Projective Plane
    TROPICAL ENUMERATION OF CURVES IN BLOWUPS OF CP 2 BRETT PARKER Abstract. We describe a method for recursively calculating Gromov{Witten invariants of all blowups of the projective plane. This recursive formula is different from the recursive formulas due to G¨ottsche and Pandharipande in the zero genus case, and Caporaso and Harris in the case of no blowups. We use tropical curves and a recursive computation of Gromov{Witten invariants relative a normal crossing divisor. This paper computes Gromov{Witten invariants of blowups of the projective plan from recursively computable relative Gromov{Witten invariants. Recursions calculating some such Gromov{Witten invariants are already known. G¨ottsche and Pandharipande give a recursive formula for zero-genus Gromov{Witten invariants of arbitary blowups of the plane in [6]. Caporaso and Harris in [5] show that the Gromov{Witten invariants of CP 2 relative to a line may be calculated recursively, giving a method for calculating Gromov{Witten invariants of CP 2 of any genus. This is extended by Vakil in [20] to a recursive formula for Gromov{Witten in- variants of CP 2 blown up at a small number of points, and further extended by Shoval and Shustin, then Brugall`ein [19, 3] to CP 2 blown up at 7 and 8 points respectively. Our recursion gives a formula for (arbitary-genus) Gromov{Witten invariants1 that is uniform in the number of blowups. Behind our recursion are exploded manifolds, [12], and the tropical gluing for- mula from [18]. This paper should be readable to someone not familiar with ex- ploded manifolds, however we lose some precision of language by forgoing any seri- ous use of exploded manifolds.
    [Show full text]
  • Luttinger Surgery Is Used to Produce Minimal Symplectic 4- Manifolds with Small Euler Characteristics
    j. differential geometry 82 (2009) 317-361 CONSTRUCTIONS OF SMALL SYMPLECTIC 4-MANIFOLDS USING LUTTINGER SURGERY Scott Baldridge & Paul Kirk Abstract Luttinger surgery is used to produce minimal symplectic 4- manifolds with small Euler characteristics. We construct a mini- mal symplectic 4-manifold which is homeomorphic but not diffeo- 2 2 morphic to CP #3CP , and which contains a genus two symplectic surface with trivial normal bundle and simply-connected comple- ment. We also construct a minimal symplectic 4-manifold which is 2 2 homeomorphic but not diffeomorphic to 3CP #5CP , and which contains two disjoint essential Lagrangian tori such that the com- plement of the union of the tori is simply-connected. These examples are used to construct minimal symplectic man- ifolds with Euler characteristic 6 and fundamental group Z, Z3, or Z/p ⊕ Z/q ⊕ Z/r for integers p,q,r. Given a group G presented with g generators and r relations, a symplectic 4-manifold with fundamental group G and Euler characteristic 10 + 6(g + r) is constructed. 1. Introduction In this article we construct a number of small (as measured by the Eu- ler characteristic e) simply connected and non-simply connected smooth 4-manifolds which admit symplectic structures. Specifically, we con- struct examples of: • A minimal symplectic manifold X homeomorphic but not diffeo- 2 morphic to CP2#3CP containing symplectic genus 2 surface with simply connected complement and trivial normal bundle, and a disjoint nullhomologous Lagrangian torus (Theorem 13). • A minimal symplectic manifold B homeomorphic but not diffeo- 2 morphic to 3CP2#5CP containing a disjoint pair of symplectic tori with simply connected complement and trivial normal bundle (Theorem 18).
    [Show full text]
  • 4-Manifolds (Symplectic Or Not)
    4-manifolds (symplectic or not) notes for a course at the IFWGP 2007 Ana Cannas da Silva∗ September 2007 – preliminary version available at http://www.math.ist.utl.pt/∼acannas/Papers/wgp.pdf Contents Introduction 2 1 4-Manifolds 3 1.1 intersection form .......................... 3 1.2 up to the 80’s ........................... 4 1.3 topological coordinates ....................... 6 1.4 smooth representatives ....................... 8 1.5 exotic manifolds .......................... 9 2 Symplectic 4-Manifolds 11 2.1 k¨ahler structures and co. ...................... 11 2.2 pseudoholomorphic curves ..................... 16 2.3 invariants for 4-manifolds ..................... 19 2.4 geography and botany ....................... 21 2.5 lefschetz pencils .......................... 24 References 27 ∗E-mail: [email protected] or [email protected] 1 Introduction These notes review some state of the art and open questions on (smooth) 4- manifolds from the point of view of symplectic geometry. They developed from the first two lectures of a short course presented at at the International Fall Work- shop on Geometry and Physics 2007 which took place in Lisbon, 5-8/September 2007. The original course included a third lecture explaining the existence on 4-manifolds of structures related to symplectic forms: 1st lecture – 4-Manifolds Whereas (closed simply connected) topological 4-manifolds are completely classified, the panorama for smooth 4-manifolds is quite wild: the existence of a smooth structure imposes strong topological constraints, yet for the same topology there can be infinite different smooth structures. We discuss constructions of (smooth) 4-manifolds and invariants to distinguish them. 2nd lecture – Symplectic 4-Manifolds Existence and uniqueness of symplectic forms on a given 4-manifold are questions particularly relevant to 4-dimensional topology and to mathe- matical physics, where symplectic manifolds occur as building blocks or as key examples.
    [Show full text]
  • Math 257A: Intro to Symplectic Geometry with Umut Varolgunes
    Math 257a: Intro to Symplectic Geometry with Umut Varolgunes Jared Marx-Kuo oct. 4th, 2019 9/23/19 1. The plan for the course is: (a) First 8 lectures from Alan Weinstein's \Lectures on Symplectic Manifolds" (b) Gromov Non-squeezing (follow Gromov's paper on J-holomorphic curves) (c) Symplectic Rigidity: Lagrangian manifolds (see Audin-Lalonde-Potterovich) 2. Some other resources are (a) Mc-Duff-Salamon (good reference but large) (b) Da Silva: Lectures on Symplectic Manifolds (c) Arnold: Methods of Classical Mechanics 3. So what is a symplectic manifold? It is M, a manifold, equipped with !, a non-degenerate closed two-form 4. !i is a smoothly varying anti-symmetric bilinear form/pairing on each TpM with p 2 M 0 0 0 0 (a) v; v 2 TpM =) !(v; v ) 2 R;!(v; v ) = −!(v ; v) (b) Non-degenerate: 0 0 8p 2 M; 8v 2 TpM; 9v 2 TpM s.t. !(v; v ) 6= 0 (c) Closed: d! = 0 5. The above three conditions give us some magic: around each point p 2 M, there exists coordinates s.t. x1; : : : ; xn; y1; : : : ; yn and ! = dx1 ^ dy1 + ··· + dxn ^ dyn 8p 2 TU ⊆ TM which in matrix coordinates looks like 0A 0 ::: 0 1 B0 A 0 :::C 0 1 B C s.t. A = (1) @0 :::A 0 A −1 0 0 ::: 0 A contrast this with a metric, which is a 2-form but has no such expression 6. Why do we study symplectic forms? (a) Answer 1: They arise naturally in many different areas of mathematics i.
    [Show full text]
  • Some Open Questions About Symplectic 4-Manifolds, Singular Plane Curves, and Braid Group Factorizations
    SOME OPEN QUESTIONS ABOUT SYMPLECTIC 4-MANIFOLDS, SINGULAR PLANE CURVES, AND BRAID GROUP FACTORIZATIONS DENIS AUROUX Abstract. The topology of symplectic 4-manifolds is related to that of singular plane curves via the concept of branched covers. Thus, various classi¯cation problems concerning symplectic 4-manifolds can be refor- mulated as questions about singular plane curves. Moreover, using braid monodromy, these can in turn be reformulated in the language of braid group factorizations. While the results mentioned in this paper are not new, we hope that they will stimulate interest in these questions, which remain essentially wide open. 1. Introduction An important problem in 4-manifold topology is to understand which manifolds carry symplectic structures (i.e., closed non-degenerate 2-forms), and to develop invariants that can distinguish symplectic manifolds. Ad- ditionally, one would like to understand to what extent the category of symplectic manifolds is richer than that of KÄahler (or complex projective) manifolds. For example, one would like to identify a set of surgery operations that can be used to turn an arbitrary symplectic 4-manifold into a KÄahler manifold, or two symplectic 4-manifolds with the same classical topological invariants (fundamental group, Chern numbers, ...) into each other. Similar questions may be asked about singular curves inside, e.g., the complex projective plane. The two types of questions are related to each other via symplectic branched covers. A branched cover of a symplectic 4- manifold with a (possibly singular) symplectic branch curve carries a natural symplectic structure. Conversely, every compact symplectic 4-manifold is a branched cover of CP2, with a branch curve presenting nodes (of both orientations) and complex cusps as its only singularities.
    [Show full text]